1. Field of the Invention
The present invention relates to a method of controlling an electric vehicle driven by an internal combustion engine, and particularly relates to an internal combustion engine driven electric vehicle in which a generator is driven by an internal combustion engine so that wheel-driving induction motors are driven by an AC output of the generator through power converters.
2. Discussion of the Related Art
In a vehicle for a large-sized construction machine such as a large-sized dump truck, a self-running-type crane car, etc., it is desired that instruments to be mounted on the vehicle be small in size and light in weight, so that the maintenance of the instruments is easy, and further that a continuous non-mechanical (electric) speed-suppressing brake is obtained for continuously coming down along a slope. To this end, an electric driving means in which an AC generator is driven by an internal combustion engine and an output of the AC generator is converted by a semiconductor power converter into AC power having a variable voltage and a variable frequency to thereby drive induction motors connected to wheels has been used more widely than a conventional mechanical driving means in which power generated by an internal combustion engine is applied to wheels through a clutch, a reduction gear, and a differential gear to thereby drive the wheels.
FIG. 4 is a schematic diagram of a main circuit of a conventional internal combustion engine driven electric vehicle arranged such that wheels are driven by three-phase induction motors. In the diagram, a synchronous generator 2 is mechanically connected to an internal combustion engine 1 such as a diesel engine, a gasoline engine, or the like, and an AC output of the synchronous generator 2 is converted into a DC power by a diode rectifier 3 used as a first power converter. An inverted-L type filter having a filter reactor 4 and a filter capacitor 5 is connected to a DC intermediate circuit provided between the DC side of the diode rectifier 3 and each of GTO inverters 8L and 8R, which will be described later, so as to smooth the DC power generated from the diode rectifier 3. There is a case in which the inverted-L type filter is omitted when the generator 2 is caused to have the function of reactor 4, and the inverters 8L and 8R are made to have the function of the capacitor 5.
The DC power smoothed through the inverted-L type filter is applied to the GTO inverters 8L and 8R which have, for example, GTO (gate turn off) thyristors. The inverters 8L and 8R are used as second power converters, so as to convert the smoothed DC power into AC power. Of the GTO inverters 8L and 8R, one inverter 8L feeds the AC power to an induction motor 9L for driving left wheels, while the other inverter 8R feeds the power to an induction motor 9R or driving right wheels.
The inverters 8L and 8R convert the smoothed DC power described above into the AC power having a variable voltage and a variable frequency through PWM operation or the like and feeds the AC power to the induction motors 9L and 9R respectively, so that the rotation speed and torque of the induction motors 9L and 9R, that is, the running speed and traction torque of a vehicle are controlled under the reverse conversion (DC-to-AC conversion) operation by the inverters 8L and 8R.
Further, the inverter 8L and the induction motor 9L for left wheels and the inverter 8R and the induction motor 9R for right wheels can be individually controlled, so that the vehicle can smoothly run on a curved running road, and even if slip is generated between the wheels and the ground, the slip can be immediately eliminated by controlling the torque and rotation speed of the wheels.
Further, in FIG. 4, a serial circuit in which a braking resistor 6 and a switch 7, which is closed in braking, are connected in series is connected between positive and negative poles of the DC intermediate circuit. When the vehicle is to be braked, the induction motors 9L and 9R are operated as the induction generators respectively, so that the AC power generated by the induction generators is converted into DC power through the forward conversion (AC-to-DC conversion) operation and the DC power is fed to the DC intermediate circuit. The DC power is consumed by the braking resistor 6 through the switch 7, so that the speed of the vehicle is suppressed through a kind of dynamic braking.
At that time, the adjustment of the power consumed by the braking resistor 6 is achieved by controlling the inverters 8L and 8R.
In the internal combustion engine driven electric vehicle shown in FIG. 4, the voltages of the induction motors 9L and 9R are controlled through the variable-voltage variable-frequency (VVVF) operation by the PWM running or the like of the inverters 8L and 8R, so that the inverters 8L and 8R generate large loss, and the power conversion efficiency is low. Accordingly, there is a disadvantage that the internal combustion engine 1 which is a driving source consumes a large quantity of fuel, so that fuel cost is high.
Further, because various kinds of instruments are mounted in a limited space in this kind of an internal combustion engine driven electric vehicle, it is required that: the driving system can be reduced in size and weight of instruments; the instruments per se are small in size and light in weight; and the whole of the system is high in performance and in efficiency and is inexpensive in cost. Specifically, because it is necessary that the induction motors 9L and 9R are accommodated in the wheels, it becomes more necessary that those induction motors are reduced in size as well as in weight.
FIG. 5 shows a characteristic of a conventional induction motor. Since the driving source of the motor is an internal combustion engine, it is desired that the motor be driven by a constant output in a vehicle speed not lower than V.sub.1. The voltage E.sub.M applied to the induction motor becomes constant in a vehicle speed range not lower than V.sub.2 as shown in the drawing. Accordingly, if the vehicle speed becomes higher than V.sub.2, the magnetic flux .phi. of the induction motor is reduced in inverse proportion to the speed, and stalling torque T.sub.d which is maximum overload torque of the induction motor is reduced in inverse proportion to a square of the speed. The torque generated by the induction motor cannot be made to be higher than this stalling torque.
Further, the size of the iron core of the inductor motor is generally determined depending on the size of the stalling torque generated in a vehicle speed range of from zero to V.sub.2 as shown in FIG. 5. In order to make the size of an induction motor small, it will do to make the stalling torque small. In a high vehicle speed range (in the range higher than V.sub.3 in the drawing), however, the stalling torque T.sub.d becomes smaller than required torque T so that the output P becomes lower than a constant output P' to make the high-speed performance of the vehicle lower.
Therefore, in order to maintain the constant output P' even in the high speed range it is necessary that the required torque T is made to be as shown by the two dots portion of line T' in the drawing in the vehicle speed range higher than V.sub.3. Accordingly, it becomes necessary to use a motor having large stalling torque as shown by T.sub.d'. In other words, the iron core of the motor to be used becomes large so that the motor becomes large in size as well as weight, resulting in a significant problem as a vehicle driving motor.
Next, braking of the internal combustion engine driven electric vehicle will be described. In this kind of vehicle, the power generated in braking is consumed by the braking resistor 6 connected to the DC intermediate circuit as described above. The power consumption P.sub.B is expressed by P.sub.B =E.sub.d.sup.2 /R, where E.sub.d represents the voltage of the DC intermediate circuit, and R represents the resistance value of the braking resistor 6.
In the conventional braking system, therefore, in order to make the braking power correspond to the operation of the vehicle, it is necessary to control the voltage E.sub.d because the resistance value R is constant in the above expression.
Accordingly, the characteristic becomes so that the braking power is reduced even if the voltage E.sub.d is constant particularly in the high vehicle speed range (in the high vehicle speed range higher than V.sub.3 in FIG. 5) to thereby make it difficult to obtain stable braking, and, sometimes, make it impossible to perform braking. This is a significant problem of braking performance. It has been therefore required to provide a braking method in which stable braking force can be obtained all over the operation range. Further, the braking resistor 6 consumes much power to generate much heat, and it has been therefore required to provide an efficient cooling method.
On the other hand, this kind of vehicle requires various large-capacity power sources, for example, a excitation source for a generator, a gate source for GTO thyristors constituting an inverter, a power source for a control device, etc. though not shown. Being large in capacity, those power sources are difficult to be provided by batteries. It is therefore desirable that the power sources are obtained efficiently and stably from a generator driven by an internal combustion engine.